Electrophotographic copying apparatus including transfer hold-down pump

Abstract

An electrophotographic copying apparatus has an electrically charged toner of one polarity which is transformed from an oppositely electrically charged surface of a photoconductive drum to the front side of a blank sheet where the front side of the blank sheet is intended to be a copy. The back side of the blank sheet engages the outer surface of a transfer drum which is positioned adjacent to the photoconductive drum. The transfer of the toner occurs within a nip defined by the photoconductive drum and the transfer drum. A cam rotates in a timed sequence relative to the transfer drum and with a vacuum pump located adjacent to the transfer drum the cam is in communication with the surface of the transfer drum so that a vacuum is created at the back side of the blank sheet resulting in the blank sheet being held to the surface of the transfer drum by ambient air pressure and the vacuum thus overcoming the electrostatic attraction of the blank sheet to the photoconductive drum.

What is claimed: 
     
       1. In an electrophotographic copying apparatus in which electrically charged toner of one polarity is transferred from an oppositely electrically charged surface of a photoconductive drum to the front side of a blank sheet for transforming said front side of the blank sheet into an intended copy as the back side of the blank sheet engages the surface of a transfer drum positioned adjacent said photoconductive drum, the transfer of toner occurring within a transfer nip defned by said photoconductive drum and said transfer drum, the improvement comprising a cam rotating in a timed sequence relative to said transfer drum,   a vacuum pump located adjacent said transfer drum and actuated by said cam and in communication with the surface of said transfer drum to create a vacuum at said back side of the blank sheet, whereby said paper is held to the surface of said transfer drum by ambient air pressure and said vacuum, overcoming the electrostatic attraction of the blank sheet to said photoconductive drum.   
     
     
       2. The improvement according to claim 1 wherein the surface of said cam comprises (a) a first portion for causing a high initial vacuum;   (b) a second portion for slowly increasing said vacuum; and   (c) a third portion for releasing said vacuum, whereby all three segments of an operational cycle to attract and release the blank sheet from the surface of said transfer drum are completed within one rotation of said cam.   
     
     
       3. The improvement according to claim 1 wherein said transfer drum includes a plurality of holes aligned across the surface of said transfer drum substantially perpendicular to the direction of travel of the blank sheet, and means for providing communication between said plurality of holes and a chamber in said vacuum pump defined by a cylinder and a piston reciprocating in said cylinder in response to movement of said cam. 
     
     
       4. The improvement according to claim 3 wherein said cam is mounted on a shaft rotating in synchronism with the surface of said photoconductive drum to insure that said cam rotates in synchronism with the holes on the surface of said transfer drum. 
     
     
       5. The improvement according to claim 2 including an angle arm connected at one end to a piston on said vacuum pump to reciprocate said piston, the other end of said angle arm having a wheel rolling on the surface of said cam to define the portions of the operational cycle of said vacuum pump in creating and releasing a vacuum. 
     
     
       6. The improvement of claim 5 wherein said cam comprises a first nearly radial surface for defining the first portion of said operational cycle, a second inclined surface for defining the second portion of said operational cycle and a third return surface for defining a third portion of said operational cycle. 
     
     
       7. The improvement of claim 1 wherein means for communicating between said vacuum pump and said surface of the transfer drum comprises a vacuum manifold running substantially perpendicular to the direction of travel of the blank sheet and having openings to the surface of said transfer drum, a hollow shaft rotating with said transfer drum and open to a vacuum chamber defined by a cylinder, and a connection between said hollow shaft and said vacuum manifold for conveying the vacuum from said vacuum pump to the surface of said transfer drum. 
     
     
       8. The improvement of claim 7 wherein said hollow shaft comprises a section of a reduced cross sectional area defining a groove in constant communication with said vacuum chamber. 
     
     
       9. In an electrophotographc copying apparatus in which electrically charged toner of one polarity is transferred from an oppositely electrically charged surface of a photoconductive drum to the front side of a blank sheet for transforming said front side of the blank sheet into an intended copy as the back side of the blank sheet engages the surface of a transfer drum positioned adjacent said photoconductive drum, the transfer of toner occurring within a transfer nip defined by the photoconductive drum and said transfer drum, the improvement comprising vacuum means for providing a vacuum to a row of holes aligned in a row across the surface of said transfer drum substantially perpendicular to the direction of travel of the blank sheet to attract the blank sheet to said transfer drum,   and timing means for controlling said vacuum means in at least three stages of operation,   a first stage providing a sharply increasing vacuum,   a second stage providing a slowly increasing vacuum, and   a third stage for releasing said vacuum and returning said timing means to its initial position.   
     
     
       10. The improvement of claim 9 wherein said vacuum means for providing a vacuum comprises a pump mounted externally of and stationary relative to said transfer drum. 
     
     
       11. The improvement of claim 9 wherein said timing means comprises a cam mounted for synchronous rotation with said transfer drum and having surfaces of varying slopes to define said stages, said vacuum means comprising a pump mounted in a stationary position relative to said transfer drum, and an angle arm having one end connected to a piston of said pump and the other end having a wheel rolling on the surface of said cam, whereby the position of said wheel and the stages of operation are controlled by the position of said wheel on said cam surface.   
     
     
       12. The improvement of claim 11 including spring means normally biasing said piston into a closed position relative to a cylinder of said pump whereby no vacuum is created. 
     
     
       13. The improvement of claim 12 including a manifold on the transfer drum and open to the surface of the transfer drum through said row of holes, a hollow shaft turning with said transfer drum, a bore in said hollow shaft before connected to said manifold, said hollow shaft extending into the cylinder of said transfer drum wherein said bore communicates with a vacuum chamber of said pump. 
     
     
       14. The improvement of claim 13 wherein said hollow shaft includes a section of a reduced cross sectional area defining a groove in constant communication with said vacuum chamber.